Despite sensitive radioimmunoassay tests for HBsAg applied to all donor blood, some type B hepatitis continues to occur following transfusion. It has previously been suggested that HBsAg-negative individuals that have anti-core antibody may transmit the hepatitis B virus. The availability of a new sensitive radioimmunoassay for anti-core antibody allows this postulate to be tested in our prospectively followed open heart surgery patients. Current tests also allow for determinations of IgM versus IgG anti-core antibody and the clinical significance of these antibody subclasses will also be investigated. In addition, it has been suggested that there is a relationship between anti-core antibody in the donor and non-A, non-B hepatitis in the recipient. This relationship will be examined in our prospectively collected samples.